copy download firmware and relock file into the flashtool installed location firmware folder. Run flashtool Administrator power off the phone, now press and hold the volume down key at the same time connect the phone to computer and release the volume down key. Click flashtool ( thunder ) symbol and mode selector window tick ( mode selector ) and firmware select the relock file and click ok. wait a few second to complete the installation after it complete using above same process flashtool download stock firmware or you can install the latest via Sony Update service after installation is complete disconnect the usb and power on

Finally you have to dial *#*#7378423#*#* in your keypad and go to service>Configuration> Under the root status if there is ( Bootloader Unlock Allowed : yes ) it mean your phone is locked bootloader.